River regulation and its influence on organic carbon dynamics, zooplankton community structure and the early life history of Murray cod in selected temperate floodplain rivers

Abstract

The impacts of instream reservoirs on the origins, transformation and fate of organic resources in rivers are largely unknown. In this thesis I compare longitudinal patterns in organic carbon character and concentration (Chapter 3) and zooplankton community structure (Chapter 4) between a free-flowing and a flow regulated temperate floodplain river and investigate the influence of river regulation on carbon sources and abiotic factors supporting the growth of larval Maccullochella peelii, Murray cod (Chapter 5). Finally, these results are modelled in a conceptual framework (Chapter 6). Results demonstrate changes in character from fresh humic and microbial DOM above the reservoir, to hydrophilic DOM below the reservoir on the Broken River. Conversely, DOM showed gradual longitudinal change in the Ovens River and was dominated by fresh humic and microbial DOM and less degraded DOM (Chapter 3). Abundances of zooplankton were higher in benthic habitats than pelagic and increased longitudinally, though decreased below the reservoir on the Broken River. Both Benthic and pelagic communities responded significantly to temperature, discharge and DOM concentration. Additionally, Benthic communities responded to peak C:M and peak C:T fluorescence ratios. Inferred feeding methods of benthic zooplankton based on their response to fluorescence ratios found the Broken River proportionally greater in taxa feeding allochthonously than the Ovens River (Chapter 4). Murray cod growth rate was higher in free-flowing/lower regulation rivers than heavily regulated rivers and decreased from endogenous to exogenous feeding stages. Additionally, interaction between these factors was significant and growth rate increased with water temperature. Differences in δ13C and δ15N occurred between catchments and a trend for increasing terrestrial contributions to bulk carbon was found with increasing flow regulation (Chapter 5). Shifts seen towards degraded allochthonous carbon in flow regulated systems has affected the dynamic nature of biofilms, the continuity of zooplankton communities and potentially the feeding breadth and growth rates of drifting larval Murray Cod. As significant descriptors of these changes for many taxa, modelling temperature, discharge and organic carbon character as ‘abiotic waves’ through time may provide insights into the occurrence of optimal condition windows for these taxa across an annual river wave (Chapter 6)